About Australian Opal — Formation, Types & Origins
Opal is Australia's national gemstone — and after 30+ years cutting, polishing and dealing in it, it still surprises me. This page covers the basics: what opal actually is, how it formed, where it comes from, and the difference between the main types. Written for anyone weighing up their first opal or wanting to understand what they're holding.
What is Opal?
Opal is from the Greek "Opallos" — to see a change of colours. It's one of the world's rarest gemstones, made up of microscopic silica spheres that diffract light into the play-of-colour you see when the stone moves. Opal contains roughly 5–9% water and rates 5.5–6.5 on the Mohs scale of hardness — softer than sapphire, harder than pearl. Treat it well and it'll outlast you.
The first opals on record were found in the 18th century on the border of Hungary and what is now the Czech Republic. Those original specimens are still on display in the museum in Vienna. Australia took over as the world's primary source in the late 1800s and hasn't looked back.
How Opal Forms
Opal needs very specific geology and climate to form, which is why it's only found in a handful of places worldwide.
Around 65–140 million years ago, during the Cretaceous period, much of Central Australia was covered by a giant inland sea. Mineral sands rich in silica washed onto its shores. The sea eventually receded and formed what we now call the Great Artesian Basin. Around 30–40 million years ago, weathering dissolved some of that silica into a gel solution, which seeped into cracks and crevices underground. Over millions of years it hardened — into opal.
One side-effect: opalised fossils. Dinosaur bones, shells and even whole creatures from that era have been found turned to opal in the same rock layers. They're some of the rarest collector pieces on earth.
Where Australian Opal Comes From
Australia produces around 95% of the world's opal. The remaining 5% comes from small deposits in Mexico, Brazil, Ethiopia and a few other countries. In late 2008, NASA even announced opal deposits on Mars — make of that what you will.
Within Australia, the four main fields each produce a different style of opal:
- Lightning Ridge, NSW — black opal country
- Coober Pedy & Mintabie, SA — white and crystal opal
- White Cliffs, NSW — historic white opal field
- Queensland (Quilpie, Yowah, Winton) — boulder opal
Types of Australian Opal
Black Opal
The rarest and most valuable type, mined almost exclusively at Lightning Ridge. It has a dark body tone — from dark grey to jet black — which makes the colour-play appear more vivid and saturated than in any other opal. Strong red or green flash on a dark body is the holy grail. Body tone is graded N1 (jet black) through N9 (crystal clear).
Boulder Opal
Boulder opal forms as veins of precious opal inside ironstone matrix in the Queensland fields — primarily Quilpie, Yowah and Winton. Unlike black opal, the opal layer can't be separated from its host rock; it's cut as a single solid piece, with the natural ironstone remaining as backing. That natural backing intensifies the colour the same way a dark body tone does in black opal. Boulder opal is also physically more durable than other opals, since the ironstone host is harder than the opal layer it carries.
Browse Boulder Opal Jewellery →
White & Crystal Opal
White opal has a light body tone — milky to translucent — and is the most widely produced type, mainly from Coober Pedy and Mintabie. Crystal opal is transparent or near-transparent, with colour-play visible through the body of the stone rather than off the surface. Both are beautiful and both have their place, but the colour reads differently to a black or boulder stone.
Opal Doublets
A doublet is a thin slice of natural Australian opal cemented to a dark backing. It's a legitimate technique used in the trade for over a century — the opal layer is real, just used efficiently so more people can own genuine Australian opal at an accessible price.
Browse Opal Doublet Jewellery →
Birthstone & Symbolism
Opal is the traditional birthstone for October, and the gemstone associated with the star signs Libra and Scorpio. There's old folklore claiming opal brings bad luck — that came from a Walter Scott novel in the 1800s and has no basis in fact. Australians have worn opal for over a century with no shortage of luck.
About Joseph
I've been in the opal trade for 30+ years — Polish-born, Australian-raised, sourcing direct from Lightning Ridge, the Queensland fields and Coober Pedy. Every stone in the shop has been through my hands. I cut and polish in my Sydney workshop, and my Sydney jewellers set the finished stones in sterling silver and 18K gold. You can find me at The Rocks Market, Sydney — under the main canopy, every Saturday and Sunday, 10am to 5pm.